Generally, an IEEE 802.16 system adds functions to a conventional air interface, and improves a conventional scheme, such that the IMT-Advanced system is implemented. In this case, a new system must maintain a conventional system without any change. Therefore, basic requirements of the new system are affected by characteristics of the conventional system.
FIG. 1 shows an IEEE 802.16 frame structure.
In order to allow a legacy terminal to access a system using the structure of FIG. 1, a preamble and an FCH must be discriminated and the discriminated result must be readily understood. Also, more MAP information must be read for a communication mode, and information needed for this reading action is contained in the FCH.
Therefore, all of the legacy terminal and a new system's terminal must read a conventional preamble and the FCH, and this reading operation can be implemented by system parameters prescribed in a conventional legacy system. That is, the new system based on the legacy maintains shortcomings of the conventional system, for example, a coverage shortage, information shortage, etc.
The following Table 1 represents contents indicated by the FCH.
TABLE 1SyntaxSize (bit)NotesDL_Frame_Prefix_Format( ){—— Used subchannel bitmap6Bit#0: Subchannel group 0Bit#1: Subchannel group {grave over ( )}Bit#2: Subchannel group 2Bit#3: Subchannel group 3Bit#4: Subchannel group 4Bit#5: Subchannel group 5 Reserved1Shall be set to zero Repetition_Coding_Indication20b00: No repetition coding on DL_MAP0b01: Repetition coding of 2 used on DL_MAP0b10: Repetition coding of 4 used on DL_MAP0b11: No repetition coding of 6 used onDL_MAP Coding_Indication30b000: CC encoding used on DL_MAP0b001: BTC encoding used on DL_MAP0b010: CTC encoding used on DL_MAP0b011: ZT CC encoding used on DL_MAP0b100: CC encoding with optional interleaver0b101: LDPC encoding used on DL_MAP0b110 to 0b111: Reserved DL_Map_Length8— Reserved4Shall be set to zero}——
With reference to Table 1, ‘FCH’ represents which one of a channel coding or a repetition coding has been used by the next MAP.
As can be seen from Table 1, a basic data bit is repeated twice in the FCH defined as each segment, a channel code (i.e., a channel code) uses a 1/2 convolutional code (133,171), and this 1/2 convolutional code (133,171) is repeatedly coded four times. Therefore, a total code rate (i.e., code rate) is set to 1/16, such that this code rate of 1/16 is considered to be very robust. However, this convolutional code has a poor performance, such that a method for improving this poor performance is needed. That is, an actual coding gain (i.e., coding gain) is obtained by the 1/2 convolutional code, and a slope of a BER graph is decided by a minimum distance of this 1/2 convolutional code. A gain corresponding to 1/4 (i.e., a gain caused by repetition) is 6 dB, such that a performance is shifted to a lower part by an SNR of 6 dB. The bit repetition produces a gain of 1.5 dB˜3 dB according to a decoder type. However, the repetition code is considered to be very improper for obtaining a channel coding gain. Therefore, a method for enabling the system to strongly transfer the FCH to a terminal is decided according to a difference between FCH construction methods.
If system coverage increases, FCH coverage enhancement is needed, and reliability of the decoded result is needed.